In 2010, it was reported that fossil fuels accounted for 80-90% of global energy consumption, and they will continue to be the predominant source of energy for the foreseeable future, considering that they are still the most abundant and affordable source of energy. Rapid economic growth in developing countries such as China and India will further amplify the increasing demand for fossil fuels. Unfortunately, fossil fuel resources are not uniformly distributed in the world, and thus many nations depend on importation for much of their fuel supply. The utilization of fossil fuels also results in the emission of many environmentally detrimental byproducts, including greenhouse gases. Therefore, the issues of energy security and imbalances in the global carbon cycle brought about by anthropogenic carbon emissions have prompted much investigation into new sustainable fuel and energy generation paradigms. Achieving a sustainable energy pathway requires both a multifaceted technological solution and the use of various energy sources. In particular, the development of efficient energy conversion schemes is desired for alternative feedstocks, rather than simply applying conventional fossil energy conversion technologies to them.
As an alternative energy resource, biomass is a feedstock that is renewable, carbon neutral, diverse, and diffusely spread throughout the world. In the United States, the U.S. Energy Information Administration (EIA) predicts that energy consumption from biomass will increase 2.9% annually from the period of 2009 to 2035, comprising 4.6% of U.S. energy consumption by 2035. For the developing world, which the EIA is projecting to have an 84% increase in energy demand versus a 14% increase for the developed world by 2035, biomass is a crucial energy resource. In 2001, nearly 50% of Africa's total primary energy supply was from biomass and waste. Biomass will therefore be an important energy feedstock for decades to come; however, it must be utilized in a sustainable and efficient manner.
As biomass is a very low energy density feedstock, thermochemical pathways have been developed to increase its energy density. One pathway is through the conversion of biomass to biocrude via pyrolysis. Biomass feedstocks can also be converted into a synthesis gas, i.e., carbon monoxide and hydrogen, through conventional or supercritical gasification processes, the latter being more well-suited to biomass feedstocks with greater than 35 wt % moisture content. Fischer-Tropsch synthesis can then be employed to make hydrocarbon fuels from the synthesis gas. Most of these thermochemical processes can be made to be highly flexible, allowing for a range of fuels to be made from a wide variety of biomass feedstocks. However, there has been less investigation into processes where biomass can be utilized as a feedstock in a local, distributed generation scheme, one that does not require increasing the energy density of the feedstock through fuel conversion to make fuel transportation feasible. Distributed biomass conversion is particularly attractive for the developing world and rural communities, as many of these regions lack the infrastructure necessary for a large scale grid. The aforementioned thermochemical conversion technologies, such as gasification and pyrolysis, can also be scaled down into small units, but due to their high operating temperatures and pressures, the main difficulties of their distributed small-scale deployment will lie in the need for skilled operators and the issue of safety. Therefore, the development of a biomass conversion scheme that can safely be operated at lower temperature and pressure is desired.
Several studies have been conducted to investigate one-step hydrogen production methods from biomass primarily through the addition of alkaline and alkaline earth hydroxides, which transfer the carbon in the biomass to a stable, solid carbonate while producing hydrogen. Thus, unlike gasification and pyrolysis where both carbon and hydrogen remain in the fuel streams, this technology allows for inherent carbon management by fixing carbon in a solid carbonate matrix while maximizing hydrogen production. Unfortunately, known processes involve an energy-intensive pretreatment process to improve mass transfer during the reaction, which entails the impregnation of an aqueous NaOH solution onto biomass, followed by the evaporation of excess water. Therefore, the overall energetics of the biomass conversion is not sustainable.
Others have investigated hydrogen conversion from cellulose using an ionic catalyst containing a base. These solid-solid cellulose systems achieved as high as 60% hydrogen conversion; however, with this approach, carbon monoxide concentration in the gaseous product stream was as high as 700 ppm under similar reaction conditions. While these types of schemes do not require the need for the aqueous NaOH solution-based pretreatment process, their catalyst preparation step did necessitate the removal of water. It was reported that greater conversions to hydrogen are observed as the sodium content in the catalyst is increased.
Another known technology converts biomass to hydrogen but requires a NaOH solution and the subsequent removal of water from the system. The removal of water is very energy intensive so that overall it would not be environmentally sustainable.